Metastatic prostate cancer remains a highly lethal disease with no curative therapeutic options. A significant subset of patients with prostate cancer harbor either germline or somatic mutations in DNA repair enzyme genes such as BRCA1, BRCA2, or ATM. Emerging data suggest that drugs that target poly(adenosine diphosphate [ADP]–ribose) polymerase (PARP) enzymes may represent a novel and effective means of treating tumors with these DNA repair defects, including prostate cancers. Here we will review the molecular mechanism of action of PARP inhibitors and discuss how they target tumor cells with faulty DNA repair functions and transcriptional controls. We will review emerging data for the utility of PARP inhibition in the management of metastatic prostate cancer. Finally, we will place PARP inhibitors within the framework of precision medicine–based care of patients with prostate cancer.

In 2016, prostate cancer is expected to be diagnosed in 180,890 men, and 26,120 will die of metastatic disease.[1] While the majority of localized prostate cancers can be controlled with surgery and/or radiation, metastatic disease remains a lethal disease with no curative options. Moreover, prostate cancer is a heterogeneous disease that can be highly lethal but also slow and indolent, as reflected by a 10-year estimated survival of 17% (S9346 trial, unpublished data). The advent of affordable and efficient techniques for profiling tumors molecularly represents an unprecedented opportunity to better characterize the molecular factors that result in indolent and/or lethal disease and to tailor therapy accordingly. Many clinical trials are already underway to examine whether molecularly targeted therapies can improve outcomes.[2] In this review, we will specifically examine the molecular rationale for one of these targeted approaches, poly(adenosine diphosphate [ADP]–ribose) polymerase (PARP) inhibition, in prostate cancer. We will review how PARP inhibitors function as a class, review the molecular features that sensitize cancer cells to this therapy, and discuss the data supporting its potential for patients with prostate cancer. We will then outline a strategy for further development of PARP inhibitors in the prostate cancer field.

Metastatic prostate cancer is typically categorized as hormone-sensitive prostate cancer (HSPC), which responds to androgen ablation, or castration-resistant prostate cancer (CRPC), which develops resistance to gonadal suppression. Although bilateral orchiectomy is the historic gold-standard treatment for metastatic HSPC, gonadal suppression is currently accomplished with gonadotropin-releasing hormone agonists or antagonists with or without androgen receptor blockade. This approach remains the cornerstone of therapy for men with metastatic HSPC.[3] Emerging data from large phase III trials (CHAARTED and Systemic Therapy in Advancing or Metastatic Prostate Cancer: Evaluation of Drug Efficacy [STAMPEDE]) have also revealed a large survival benefit for the combination of docetaxel and androgen deprivation in metastatic HSPC.[4,5]

Despite these initially effective treatments, the vast majority of men with metastatic HSPC will progress to CRPC, which is the lethal stage of the disease. For these patients, several additional therapies provide benefit by further suppression of androgen signaling (enzalutamide, abiraterone), disruption of the cell cycle in replicating cells (docetaxel, cabazitaxel), targeting of bone metastases (radium-223), or activation of antitumor immunologic response (sipuleucel-T).[6] While these therapies have undoubtedly extended the median survival of patients with metastatic CRPC, their impact on survival is modest and they clearly do not work for all men. In addition, we lack validated genomic markers that would allow better selection of patients for these therapies. Therefore, a better approach that leverages the individual and unique aspects of a patient’s cancer and utilizes therapy based on these factors may allow us to improve patient outcomes.

The development of high-throughput sequencing technology has made it feasible to comprehensively analyze the genetic mutations and gene expression changes in individual prostate cancers with a high degree of resolution in real time. Many institutions now routinely perform these analyses in the hope that they might uncover molecular features that predict response to certain therapies or provide guidance for clinical trial selection.[7] This approach, colloquially termed “precision” medicine, offers the potential promise of providing the right therapy for the right patient at the right time. In the context of prostate cancer, it means molecularly characterizing a tumor and then offering patients drugs that may specifically promote tumor lethality based on these molecular features. The limitation of this approach is that it requires that the target be truly biologically relevant and that there are drugs that can effectively target these molecular changes. The discovery of both somatic and germline DNA repair deficiencies in prostate cancer, together with the development of PARP inhibitors that can kill cancer cells with these defects, is a potent example of targeting therapy to molecularly defined tumor subtypes. While much early work validating this approach has occurred in breast and ovarian cancer populations, emerging data suggest that PARP inhibition is a potentially important strategy for managing a significant subset of prostate cancer patients.

PARP Inhibition: Targeting DNA Repair Deficiency

Molecular mechanism

PARP1 catalyzes the addition of poly(ADP)-ribose (PAR) groups to target proteins in a process termed PARylation.[8] PARP1 is part of a superfamily of proteins that consists of 18 members (including the related tankyrase enzymes), which have many functions within normal and cancer cells. PARP1, the founding member of this family, is responsible for the majority of PARylation of protein targets within cells. It is primarily present in the nucleus in association with chromatin, where it participates in DNA repair and regulation of gene expression by modulating protein localization and activity.[9]

DNA damage occurs continuously in all living cells as a result of oxidative damage or DNA replicative stress.[10] When DNA damage occurs on one strand of the DNA double helix, a single-strand break (SSB) results, but if two SSBs occur in close proximity and on opposite strands, the result is a double-strand break (DSB) and discontinuity of the chromosome (Figures 1 and 2). Even a single DSB is lethal to a human cell if unrepaired because of the risk of large-scale loss of genetic information.

PARP1 plays a critical role in restoration of genomic integrity by facilitating efficient repair of DNA SSBs and DSBs. PARP1 senses DNA damage by binding to the site of SSBs and DSBs and inducing auto-PARylation, which in turn promotes recruitment of DNA repair factors (such as DNA ligase III, polymerase β, and x-ray repair cross-complementing protein 1[XRCC1]).[11] Loss of PARP1 function by means of pharmacologic or genetic mechanisms results in impaired SSB repair and, following initiation of DNA replication, creation of a DNA DSB (see Figure 1). PARP may also play an important role in DSB repair and is known to recruit the MRE11-RAD50-NBS1 complex and to promote PARylation of BRCA1, factors required for the homologous recombination (HR) pathway of DNA DSB repair. Therefore, pharmacologic inhibition of PARP1/2 in DNA repair–defective (DRD) cells that lack efficient HR repair capabilities (such as those harboring BRCA1, BRCA2, or ATM mutations) results in failure to resolve SSBs, which are then converted to DSBs that promote cellular death.

The activity of PARP1 is not limited to DNA damage response. PARP1 is also known to regulate gene expression by modulation of transcription factor activity and regulation of chromatin.[12] PARP1 binds to RNA polymerase II, regulating gene expression, and may also affect tumor suppressor and oncogenic gene expression. PARP1 can also modulate hormone-dependent gene transcription from hormone-responsive nuclear receptors, such as estrogen receptors α and β, progesterone receptor, and androgen receptor.[9]

Furthermore, PARP1 can modulate the transcriptional activity of ETS transcription factors, which suggests that pharmacologic targeting of PARP1 may be useful in TMPRSS2:ERG fusion–positive prostate cancer cells (~50% of prostate cancers).[13] PARP1 physically interacts with the TMPRSS2:ERG gene fusion and the DNA–protein kinase complex, and these interactions are required for ERG-related gene transcription. Interestingly, PARP inhibition with olaparib inhibited prostate cancer xenograft growth if tumors harbored a TMPRSS2:ERG fusion, which suggests that PARP might represent a therapeutic option for prostate cancer patients with TMPRSS2:ERG fusions.[13] This concept is being evaluated in a recently completed clinical trial (National Cancer Institute [NCI] 9012).

PARP inhibitors

Given the biologic importance of PARP1 in the context of cancer, several pharmacologic agents that target this enzyme are currently under development (Table).

Most PARP inhibitors mimic the NAD+ substrate of PARP1, competitively bind to the catalytic domain, and inhibit PAR synthesis.[14] PARP inhibitors require the expression of PARP1 and PARP2, and cells that lack expression of both genes are not sensitive to these agents. PARP inhibitors all appear to block catalytic activity and PAR synthesis in a roughly equivalent manner but may show differential ability to trap PARP1/2 at the site of DNA damage (niraparib > olaparib > veliparib), an event that blocks repair and promotes cellular lethality.[15,16] Whether these effects observed in vitro translate into clinically meaningful differences in efficacy is less clear. Furthermore, it is also now clear that the putative PARP inhibitor iniparib may not promote cytotoxicity via PARP inhibition. Several initial studies focused on iniparib, but when phase III trials failed to demonstrate the efficacy of this compound, additional mechanistic work demonstrated that iniparib may not truly be an effective PARP inhibitor.[17,18] These data illustrate the necessity of careful mechanistic characterization of any targeted agent prior to large-scale and expensive studies.

Germline DNA repair deficiency

Inherited defects in DNA repair pathways result in increased susceptibility to the development of malignancy.[19] Defects in mismatch repair proteins promote the development of tumors, including colon and uterine,[20] whereas inherited inactivating mutations in BRCA1 and BRCA2, which are required for efficient HR-based DNA DSB repair, significantly increase the risk of breast, ovarian, prostate, and other cancers.[21] Patients with these tumor types typically demonstrate homozygous inactivation of these genes, the first event occurring in the germline, with subsequent clonal somatic inactivation of the remaining allele.[21] These events presumably occur early in tumorigenesis and, by loss of robust DNA DSB repair, induce genomic instability, which causes loss of tumor suppressors, activation of oncogenes, and acceleration of tumorigenesis.

A germline mutation in BRCA1 or BRCA2 increases the risk of prostate cancer and thus may be found in 2% to 5% of prostate cancers.[22,23] The relative risk of development of prostate cancer for men ≤ age 65 with BRCA1 mutations is 1.8, but BRCA2 mutations in particular seem to increase the risk of prostate cancer formation by age 65 by about 8.6-fold. Mutations of BRCA1, BRCA2, and ATM (and perhaps other DNA repair genes) may also play a role in progression to the lethal castration-resistant state.[22,24-26] The frequency of BRCA2 germline mutations in prostate cancer alone may be as high as 2%.[22] Therefore, the development of therapies to target DNA repair is likely to benefit a relatively large and relatively young population.

Somatic DNA repair deficiency

In addition to germline defects, tumors can acquire defective DNA repair processes through somatic loss of DNA damage response genes, and these somatic mutations can also confer sensitivity to PARP inhibition.[27] This has led to the concept of “BRCAness,” which refers to somatically acquired defects in HR that, as a group, could predict tumor response to PARP inhibitors and cisplatin.[21] Somatic alterations can include either acquired mutations or epigenetic events that silence genes such as ATM; ATR; BRCA1 or -2; CHEK1 or -2; FANCA, -C, -D2, -E, -F; PALB2; MRE11 complex; or RAD51, which prevent efficient HR repair of DNA DSBs.

It is likely that a substantial proportion of men with prostate cancer may demonstrate aspects of BRCAness that could predict sensitivity to PARP inhibitors. Beltran et al performed targeted next-generation sequencing of tumors from men with advanced prostate cancer and found that 12% demonstrated BRCA2 loss and that 8% harbored ATM loss.[28] Furthermore, up to 19.3% of CRPCs demonstrate aberrations in BRCA1, BRCA2, or ATM; these events become more frequent as the disease progresses from hormone-sensitive to castration-resistant.[29] Together these data suggest that BRCAness is a reasonably frequent event in patients with advanced prostate cancer, which makes PARP inhibition an attractive target in this disease.

Synthetic lethality

The concept of promoting the killing of cancer cells by simultaneously blocking SSB repair using PARP inhibition in cells that lack efficient DSB repair is called “synthetic lethality.” In this scenario, tumor cells may harbor either germline or somatically acquired homozygous inactivation of HR. Germline defects (when present) typically affect only one allele in normal cells, and therefore normal tissues retain HR function. This difference between the DNA repair capacity of normal and cancer cells can be leveraged to produce selective cell killing of tumor cells by PARP inhibitors. Treatment of patients with PARP inhibitors will then block normal SSB repair in all cells, and these SSBs are subsequently converted to DSBs by DNA replication. In normal cells, HR restores the genome and allows survival, but in DRD cancer cells, DSBs persist, inducing cellular death selectively in the tumor cell population (see Figure 2).

PARP Inhibition in Prostate Cancer

Early-phase studies

Ample data indicate that PARP inhibitors possess antitumor activity within diverse patient populations, particularly those with BRCA1 or BRCA2 mutations.[14] One of the first studies to validate the concept of clinical benefit in patients with BRCA mutations was a phase I trial that looked at pharmacokinetic and pharmacodynamic aspects of olaparib treatment.[24] In this study, 60 patients with solid tumors were treated with various doses of olaparib (10 mg daily to 600 mg twice daily) to determine maximum tolerated dose (MTD). The study population was intentionally enriched for BRCA mutation carriers, and 22 patients of the cohort harbored BRCA1 or BRCA2 mutations. Objective tumor activity was observed in the mutation carrier population in patients with breast, ovarian, and prostate cancers. Three patients with advanced prostate cancer were included in this study cohort; the one with a BRCA2 mutation had a greater than 50% response in prostate-specific antigen (PSA) level, resolution of bone metastases, and an extended treatment course. This study suggested that there was a benefit of olaparib therapy in BRCA mutation carriers and the potential for benefit in prostate cancer patients. Further validation of olaparib efficacy in patients with BRCA mutations came from parallel proof-of-concept studies demonstrating the activity of this agent in women with breast and ovarian cancers and BRCA1 or BRCA2 mutations.[30,31] These data ultimately led to US Food and Drug Administration (FDA) approval of olaparib for women with a BRCA mutation and metastatic ovarian cancer after chemotherapy.

Additional data that demonstrate a similar spectrum of activity are available for other PARP inhibitors. Phase I data on the safety and pharmacodynamics of single-agent veliparib have been reported as an abstract,[32] and additional studies of veliparib in combination with mitomycin,[33] irinotecan,[34] and other agents have been reported.[35] VanderWeele et al published a case report of a patient with metastatic CRPC and BRCA2 mutation who had a sustained complete response to veliparib and carboplatin/gemcitabine.[36] It seems likely that many of the available PARP inhibitors may have overlapping activities, and further data will be needed to clarify which agent to use in which tumor type and the relative toxicities of each agent.

Temozolomide and veliparib in metastatic CRPC

Compelling data implicate PARP1 in the mediation of DNA repair responses to alkylating agents,[37] cellular survival in BRCA-deficient cells,[24,38] and androgen receptor–mediated prostate cancer cellular proliferation.[9,39] Furthermore, data suggest that prostate cancers that harbor the TMPRSS2:ERG fusion (present in up to 50% of prostate cancers) may be more sensitive to PARP inhibition.[13] Therefore, Hussain et al carried out a single-arm pilot study to assess the safety and efficacy of veliparib with the alkylator temozolomide (TMZ) in patients with metastatic CRPC following docetaxel therapy.[40] In this study, patients with a PSA level of ≥ 2 ng/mL were treated with veliparib, 40 mg twice daily, on days 1 to 7 and TMZ, 150 to 200 mg/m2, on days 1 to 5 on a 28-day cycle, based on tolerance data from a phase I study (ClinicalTrials.gov identifier: NCT00526617). The primary endpoint was PSA response rate (30% decline). Of the 25 patients who were evaluable for response, 2 had a confirmed response, 13 had stable PSA, and 10 had progression. The most frequent toxicities were thrombocytopenia, anemia, fatigue, neutropenia, nausea, and constipation. The investigators did assess frequency of TMPRSS2:ERG fusion but found it in only one of eight evaluable patients. Although this patient had stable disease, no conclusions could be drawn regarding the contribution of the fusion product to veliparib sensitivity. Overall, while the combination was considered tolerable, it had only modest activity. No preselection was done in the study, and because BRCAness exists in 20% of patients, it is perhaps not surprising that activity was modest. The lower dose of PARP inhibitor and the lack of established benefit for TMZ may also have contributed to less than robust clinical activity for this combination. Given the emerging molecular data, it seems that future studies will be more likely to identify activity if done in preselected patient populations.

TOPARP

The Trial of PARP Inhibition in Prostate Cancer (TOPARP-A) sought to determine whether patients with prostate cancers with molecularly identified defects in DNA repair benefited from full-dose olaparib therapy.[25] In this phase II study, 50 men with CRPC underwent biopsy of metastatic disease and targeted next-generation sequencing, exome and transcriptome analysis, and digital polymerase chain reaction. The primary endpoint was response rate (either objective response or reduction of 50% in PSA level or reduction in circulating tumor cells). All had previously received docetaxel, and most had been treated with abiraterone or enzalutamide (98%) and cabazitaxel (58%). Patients were grouped according to the presence or absence of a homozygous deletion of or deleterious mutation in DNA damage response genes, which predict sensitivity to PARP inhibition. Overall, 16 of 49 evaluable patients (33%) were biomarker positive (indicative of homozygous deleterious changes in BRCA1/2, ATM, Fanconi anemia genes, orCHEK2). Of these, five patients had germline and somatic events (three patients with germline BRCA2 and three patients with germline ATM deletions or mutations). Of the 16 patients with deleterious changes in DNA repair genes, 14 (88%) responded to olaparib. The median overall survival for patients with biomarker-positive DRD tumors who received olaparib was 13.8 months, compared with 7.5 months for those with biomarker-negative tumors (P = .05). Interestingly, two biomarker-negative patients also met criteria for response to olaparib. Although one was a longer-term responder still on therapy at the time of publication, this particular patient did harbor monoallelic deletions of bothBRCA2 and PALB2 that did not meet criteria for the prespecified biomarker-positive category but that may have contributed to tumor sensitivity. Toxicity was as expected, with patients displaying grade 3 or 4 anemia (10/50), fatigue (6/50), leukopenia (3/50), thrombocytopenia (2/50), and neutropenia (2/50). These results illustrate the feasibility of using molecular profiling to identify prostate cancers that display molecular features suggestive of sensitivity to PARP inhibition (BRCAness).

NCI 9012

ETS gene fusions—which result from gene rearrangement and juxtaposition of an androgen-responsive gene, such as TMPRSS2, to an ETS transcription factor gene, such as ERG or ETV1—occur in 50% to 60% of prostate cancers.[41,42] ETS transcription factors may also physically interact with PARP1, and PARP1 activity may be required for ETS-mediated invasion, transcription, and metastasis.[13] Androgen receptor–mediated transcription may also promote DNA DSBs and requires PARP activity for efficient repair.[43-45] Therefore, therapeutic targeting of androgen receptor signaling and PARP1 activity using abiraterone and veliparib is an attractive strategy in the management of metastatic prostate cancer.

A randomized phase II clinical trial in patients with metastatic CRPC was recently completed; it examined whether ETS fusion is a biomarker of response to abiraterone or abiraterone plus veliparib. In this study, 148 patients with metastatic CRPC underwent biopsy followed by assessment of ETS fusion status and then random assignment to either abiraterone alone or abiraterone plus veliparib. The primary endpoint was confirmed PSA response in patients receiving either abiraterone alone or combination therapy, stratified by ETS status. Secondary endpoints included safety, objective response rate, progression-free survival, and whether DNA repair gene deficiency (homozygous deletions of or deleterious mutations in: BRCA1, BRCA2, ATM, FANCA, PALB2, RAD51B, RAD51C) predicts response. This trial has now completed enrollment, and preliminary results will be presented at the American Society of Clinical Oncology 2016 Annual Meeting. Although final results are pending, the study does illustrate the feasibility of a large-scale metastatic tissue–based, biomarker-driven trial involving PARP inhibition in patients with metastatic CRPC. This study will also begin to ascertain the role of ETS fusions in determining response to PARP inhibitor therapy and will further explore the contribution of DRD to patient outcomes in those treated with standard therapy (abiraterone arm) and those treated with PARP inhibition (abiraterone plus veliparib arm).

Future studies

Given the data from the studies discussed previously and the enthusiasm for molecularly targeted trials in oncology, there is interest in further testing of PARP inhibition in prostate cancer patients. Multiple trials have recently been completed, are actively enrolling, or are nearing activation within this space (see Table, ClinicalTrials.gov).

Olaparib. Olaparib is the agent that is farthest along in clinical development and has an FDA indication in ovarian cancer. Olaparib also has the most active or pending studies in prostate cancer patients. TOPARP continues to enroll patients with metastatic CRPC, with a target accrual of 98 patients (ClinicalTrials.gov identifier: NCT01682772). There is a randomized double-blind, placebo-controlled phase II study of abiraterone plus olaparib or placebo for patients with metastatic CRPC who received prior docetaxel therapy (ClinicalTrials.gov identifier: NCT01972217). This trial, which is similar to the NCI 9012 study, has completed enrollment, but results are pending. Another trial is examining the biologic effect of olaparib on prostate cancer specimens when given alone or in combination with degarelix prior to prostatectomy (ClinicalTrials.gov identifier: NCT02324998). Furthermore, there is an open-label phase II study to assess the efficacy and safety of olaparib in patients with BRCA1 or BRCA2 mutations (regardless of tumor type), which is ongoing but no longer enrolling patients (ClinicalTrials.gov identifier: NCT01078662).

Veliparib. NCI 9012 (discussed previously) will help determine whether veliparib has potential therapeutic activity in metastatic CRPC and may identify molecularly determined subsets of disease (ie, ETS fusion–positive, DRD-positive) that might be expected to show the most benefit. The results of this study may help determine whether additional studies of this agent within the prostate cancer space are warranted.

Niraparib. The Hoosier Cancer Research Network has a planned phase I study of the combination of enzalutamide and niraparib for patients with metastatic CRPC (ClinicalTrials.gov identifier: NCT02500901), which has not yet begun enrollment. The primary endpoint of this study will be determination of MTD and dose-limiting toxicity.

Talazoparib. Although no prostate cancer–specific trials using other PARP inhibitors are currently active, several trials for molecularly targeted patient populations or phase I trials for toxicity assessment in combination with chemotherapy are ongoing; these provide some information on prostate cancer populations, depending on the types of solid tumors enrolled. There is a phase I trial of talazoparib in combination with carboplatin and paclitaxel (ClinicalTrials.gov identifier: NCT02317874) and another for patients with solid tumors and hepatic and renal dysfunction (ClinicalTrials.gov identifier: NCT02567396).

Precision Targeting of the PARP Pathway in Prostate Cancer
PARP inhibitors are a promising therapeutic option for men with prostate cancer. There is good evidence that men with either germline or somatic mutations in DNA repair pathways can derive therapeutic benefit from inhibition of PARP1/2, which blocks repair of SSB, driving persistent DSBs that lead to cancer cell lethality. Preclinical data also suggest that PARP inhibition may produce benefits by targeting chromatin and gene transcription, which implies that clinical benefits may extend beyond patients with DRD tumors.[12] To continue to develop PARP inhibitors within the prostate cancer field, we will need to develop and refine a set of biomarkers for use in selecting the right patient populations for these agents and then incorporate these biomarkers into prospective studies. As part of a precision therapy strategy, PARP inhibitors will likely play an important role in the management of prostate cancer in the near future.

It is now feasible to comprehensively profile the mutational, epigenetic, and gene expression changes in men with prostate cancer, and we are beginning to use this information to guide treatment choices.[7] Unfortunately, the functional relevance of many of the molecular features uncovered in these profiles is not completely understood. DNA repair processes are complex and require many genes for efficient repair of various types of DNA damage. Most past and ongoing studies focused on patients with specific molecular features, such as BRCA1, BRCA2, ATM, FANCA, PALB2, RAD51B, and RAD51C mutations. While mutations of these genes are likely to affect sensitivity to PARP inhibitors, mutations in other DNA repair or transcription factor genes may as well, and identification of those genes could expand the patient population that could benefit from therapy. Determination of whether other genes are susceptible to PARP inhibitor therapy will require robust preclinical models with a wide selection of genetic changes that reflect human disease; such models can be used to determine whether additional mutations and epigenetic or gene expression changes also result in PARP inhibitor sensitivity. Given the potential infrequency of many of the individual mutations that might sensitize to PARP inhibitors, large-scale registries that catalog mutations and their responsiveness to therapies may be needed.

As we define the molecular features that suggest sensitivity to PARP inhibition, the challenge will then become understanding the best strategy for incorporating these targeted agents into our standard treatment algorithms. In the context of prostate cancer, PARP inhibitors could be considered in high-risk patient populations in an adjuvant manner, before or with androgen deprivation therapy (ADT) in patients with newly metastatic disease, or in the setting of castration-resistant disease before or after the many other therapeutic options. To date, most trials in the prostate cancer space have been in the castration-resistant setting, perhaps because mutations in DNA damage genes may become more common as the disease progresses.[25] Nonetheless, there is no reason to assume that patients who harbor mutations may not benefit earlier in the disease course. Adjuvant use of PARP inhibitors in those with high-risk or micrometastatic disease could conceivably render patients disease free. Similarly, the combination of ADT and PARP inhibitors in early metastatic disease may provoke prolonged progression-free intervals similar to the situation with early docetaxel therapy but with less toxicity.[4,5] In the context of castration-resistant disease, it is reasonable to hypothesize that the combination of PARP inhibitors with hormonal agents such as abiraterone or enzalutamide or with chemotherapies might act synergistically to promote disease control.

The trials to examine these questions may be more challenging to design and execute because patients with sensitizing molecular changes represent a limited subset of total patients with prostate cancer. This means that in order to identify the subset that will benefit, many will need to be screened.[25] Because most molecular analyses are done using biopsy tissue, screening and cost may be challenging factors. In addition, the natural history of patients with DNA damage pathway mutations may also be distinct from those without such mutations. It is conceivable that mutations in DNA damage response genes may modulate patient response to standard hormonal agents, chemotherapy, or radium because all three of these therapeutic modalities have the potential to induce DNA damage in prostate cancer cells. Given these caveats, it will be essential to design an efficient precision medicine clinical trial pipeline that can rapidly molecularly profile patient tumors, assign to a therapeutic intervention, and then assess the complex resulting data and analyze results according to molecular categories.

PARP inhibitors have the potential to be a promising addition to the therapeutic arsenal used to treat prostate cancer and other solid tumors that harbor the appropriate molecular features. The transition from a standard, one-size-fits-all approach to a targeted, precision medicine strategy in which an individual prostate cancer patient’s tumor biology will guide choice of therapy will require careful planning and thought. The inclusion of PARP-targeted therapies before, after, with, or in place of standard hormonal therapies and chemotherapies will need to be defined so as to maximize antitumor effect and patient survival. Hopefully, application of these novel combinations in those most likely to benefit will ultimately lead to longer and better lives for patients with prostate cancer.
REFERENCES

Metastatic prostate cancer remains a highly lethal disease with no curative therapeutic options. A significant subset of patients with prostate cancer harbor either germline or somatic mutations in DNA repair enzyme genes such as BRCA1, BRCA2, or ATM. Emerging data suggest that drugs that target poly(adenosine diphosphate [ADP]–ribose) polymerase (PARP) enzymes may represent a novel and effective means of treating tumors with these DNA repair defects, including prostate cancers. Here we will review the molecular mechanism of action of PARP inhibitors and discuss how they target tumor cells with faulty DNA repair functions and transcriptional controls. We will review emerging data for the utility of PARP inhibition in the management of metastatic prostate cancer. Finally, we will place PARP inhibitors within the framework of precision medicine–based care of patients with prostate cancer.

Introduction
In 2016, prostate cancer is expected to be diagnosed in 180,890 men, and 26,120 will die of metastatic disease.[1] While the majority of localized prostate cancers can be controlled with surgery and/or radiation, metastatic disease remains a lethal disease with no curative options. Moreover, prostate cancer is a heterogeneous disease that can be highly lethal but also slow and indolent, as reflected by a 10-year estimated survival of 17% (S9346 trial, unpublished data). The advent of affordable and efficient techniques for profiling tumors molecularly represents an unprecedented opportunity to better characterize the molecular factors that result in indolent and/or lethal disease and to tailor therapy accordingly. Many clinical trials are already underway to examine whether molecularly targeted therapies can improve outcomes.[2] In this review, we will specifically examine the molecular rationale for one of these targeted approaches, poly(adenosine diphosphate [ADP]–ribose) polymerase (PARP) inhibition, in prostate cancer. We will review how PARP inhibitors function as a class, review the molecular features that sensitize cancer cells to this therapy, and discuss the data supporting its potential for patients with prostate cancer. We will then outline a strategy for further development of PARP inhibitors in the prostate cancer field.
Metastatic prostate cancer is typically categorized as hormone-sensitive prostate cancer (HSPC), which responds to androgen ablation, or castration-resistant prostate cancer (CRPC), which develops resistance to gonadal suppression. Although bilateral orchiectomy is the historic gold-standard treatment for metastatic HSPC, gonadal suppression is currently accomplished with gonadotropin-releasing hormone agonists or antagonists with or without androgen receptor blockade. This approach remains the cornerstone of therapy for men with metastatic HSPC.[3] Emerging data from large phase III trials (CHAARTED and Systemic Therapy in Advancing or Metastatic Prostate Cancer: Evaluation of Drug Efficacy [STAMPEDE]) have also revealed a large survival benefit for the combination of docetaxel and androgen deprivation in metastatic HSPC.[4,5]

Despite these initially effective treatments, the vast majority of men with metastatic HSPC will progress to CRPC, which is the lethal stage of the disease. For these patients, several additional therapies provide benefit by further suppression of androgen signaling (enzalutamide, abiraterone), disruption of the cell cycle in replicating cells (docetaxel, cabazitaxel), targeting of bone metastases (radium-223), or activation of antitumor immunologic response (sipuleucel-T).[6] While these therapies have undoubtedly extended the median survival of patients with metastatic CRPC, their impact on survival is modest and they clearly do not work for all men. In addition, we lack validated genomic markers that would allow better selection of patients for these therapies. Therefore, a better approach that leverages the individual and unique aspects of a patient’s cancer and utilizes therapy based on these factors may allow us to improve patient outcomes.

The development of high-throughput sequencing technology has made it feasible to comprehensively analyze the genetic mutations and gene expression changes in individual prostate cancers with a high degree of resolution in real time. Many institutions now routinely perform these analyses in the hope that they might uncover molecular features that predict response to certain therapies or provide guidance for clinical trial selection.[7] This approach, colloquially termed “precision” medicine, offers the potential promise of providing the right therapy for the right patient at the right time. In the context of prostate cancer, it means molecularly characterizing a tumor and then offering patients drugs that may specifically promote tumor lethality based on these molecular features. The limitation of this approach is that it requires that the target be truly biologically relevant and that there are drugs that can effectively target these molecular changes. The discovery of both somatic and germline DNA repair deficiencies in prostate cancer, together with the development of PARP inhibitors that can kill cancer cells with these defects, is a potent example of targeting therapy to molecularly defined tumor subtypes. While much early work validating this approach has occurred in breast and ovarian cancer populations, emerging data suggest that PARP inhibition is a potentially important strategy for managing a significant subset of prostate cancer patients.

PARP Inhibition: Targeting DNA Repair Deficiency

Molecular mechanism

PARP1 catalyzes the addition of poly(ADP)-ribose (PAR) groups to target proteins in a process termed PARylation.[8] PARP1 is part of a superfamily of proteins that consists of 18 members (including the related tankyrase enzymes), which have many functions within normal and cancer cells. PARP1, the founding member of this family, is responsible for the majority of PARylation of protein targets within cells. It is primarily present in the nucleus in association with chromatin, where it participates in DNA repair and regulation of gene expression by modulating protein localization and activity.[9]

DNA damage occurs continuously in all living cells as a result of oxidative damage or DNA replicative stress.[10] When DNA damage occurs on one strand of the DNA double helix, a single-strand break (SSB) results, but if two SSBs occur in close proximity and on opposite strands, the result is a double-strand break (DSB) and discontinuity of the chromosome (Figures 1 and 2). Even a single DSB is lethal to a human cell if unrepaired because of the risk of large-scale loss of genetic information.

PARP1 plays a critical role in restoration of genomic integrity by facilitating efficient repair of DNA SSBs and DSBs. PARP1 senses DNA damage by binding to the site of SSBs and DSBs and inducing auto-PARylation, which in turn promotes recruitment of DNA repair factors (such as DNA ligase III, polymerase β, and x-ray repair cross-complementing protein 1[XRCC1]).[11] Loss of PARP1 function by means of pharmacologic or genetic mechanisms results in impaired SSB repair and, following initiation of DNA replication, creation of a DNA DSB (see Figure 1). PARP may also play an important role in DSB repair and is known to recruit the MRE11-RAD50-NBS1 complex and to promote PARylation of BRCA1, factors required for the homologous recombination (HR) pathway of DNA DSB repair. Therefore, pharmacologic inhibition of PARP1/2 in DNA repair–defective (DRD) cells that lack efficient HR repair capabilities (such as those harboring BRCA1, BRCA2, or ATM mutations) results in failure to resolve SSBs, which are then converted to DSBs that promote cellular death.

The activity of PARP1 is not limited to DNA damage response. PARP1 is also known to regulate gene expression by modulation of transcription factor activity and regulation of chromatin.[12] PARP1 binds to RNA polymerase II, regulating gene expression, and may also affect tumor suppressor and oncogenic gene expression. PARP1 can also modulate hormone-dependent gene transcription from hormone-responsive nuclear receptors, such as estrogen receptors α and β, progesterone receptor, and androgen receptor.[9]

Furthermore, PARP1 can modulate the transcriptional activity of ETS transcription factors, which suggests that pharmacologic targeting of PARP1 may be useful in TMPRSS2:ERG fusion–positive prostate cancer cells (~50% of prostate cancers).[13] PARP1 physically interacts with the TMPRSS2:ERG gene fusion and the DNA–protein kinase complex, and these interactions are required for ERG-related gene transcription. Interestingly, PARP inhibition with olaparib inhibited prostate cancer xenograft growth if tumors harbored a TMPRSS2:ERG fusion, which suggests that PARP might represent a therapeutic option for prostate cancer patients withTMPRSS2:ERG fusions.[13] This concept is being evaluated in a recently completed clinical trial (National Cancer Institute [NCI] 9012).

PARP inhibitors

Given the biologic importance of PARP1 in the context of cancer, several pharmacologic agents that target this enzyme are currently under development (Table). Most PARP inhibitors mimic the NAD+ substrate of PARP1, competitively bind to the catalytic domain, and inhibit PAR synthesis.[14] PARP inhibitors require the expression of PARP1 and PARP2, and cells that lack expression of both genes are not sensitive to these agents. PARP inhibitors all appear to block catalytic activity and PAR synthesis in a roughly equivalent manner but may show differential ability to trap PARP1/2 at the site of DNA damage (niraparib > olaparib > veliparib), an event that blocks repair and promotes cellular lethality.[15,16] Whether these effects observed in vitro translate into clinically meaningful differences in efficacy is less clear. Furthermore, it is also now clear that the putative PARP inhibitor iniparib may not promote cytotoxicity via PARP inhibition. Several initial studies focused on iniparib, but when phase III trials failed to demonstrate the efficacy of this compound, additional mechanistic work demonstrated that iniparib may not truly be an effective PARP inhibitor.[17,18] These data illustrate the necessity of careful mechanistic characterization of any targeted agent prior to large-scale and expensive studies.

Germline DNA repair deficiency

Inherited defects in DNA repair pathways result in increased susceptibility to the development of malignancy.[19] Defects in mismatch repair proteins promote the development of tumors, including colon and uterine,[20] whereas inherited inactivating mutations in BRCA1 and BRCA2, which are required for efficient HR-based DNA DSB repair, significantly increase the risk of breast, ovarian, prostate, and other cancers.[21] Patients with these tumor types typically demonstrate homozygous inactivation of these genes, the first event occurring in the germline, with subsequent clonal somatic inactivation of the remaining allele.[21] These events presumably occur early in tumorigenesis and, by loss of robust DNA DSB repair, induce genomic instability, which causes loss of tumor suppressors, activation of oncogenes, and acceleration of tumorigenesis.

A germline mutation in BRCA1 or BRCA2 increases the risk of prostate cancer and thus may be found in 2% to 5% of prostate cancers.[22,23] The relative risk of development of prostate cancer for men ≤ age 65 with BRCA1 mutations is 1.8, but BRCA2 mutations in particular seem to increase the risk of prostate cancer formation by age 65 by about 8.6-fold. Mutations of BRCA1, BRCA2, and ATM (and perhaps other DNA repair genes) may also play a role in progression to the lethal castration-resistant state.[22,24-26] The frequency of BRCA2 germline mutations in prostate cancer alone may be as high as 2%.[22] Therefore, the development of therapies to target DNA repair is likely to benefit a relatively large and relatively young population.

Somatic DNA repair deficiency

In addition to germline defects, tumors can acquire defective DNA repair processes through somatic loss of DNA damage response genes, and these somatic mutations can also confer sensitivity to PARP inhibition.[27] This has led to the concept of “BRCAness,” which refers to somatically acquired defects in HR that, as a group, could predict tumor response to PARP inhibitors and cisplatin.[21] Somatic alterations can include either acquired mutations or epigenetic events that silence genes such as ATM; ATR; BRCA1 or –2; CHEK1 or -2; FANCA, -C, -D2, -E, -F; PALB2; MRE11 complex; or RAD51, which prevent efficient HR repair of DNA DSBs.

It is likely that a substantial proportion of men with prostate cancer may demonstrate aspects of BRCAness that could predict sensitivity to PARP inhibitors. Beltran et al performed targeted next-generation sequencing of tumors from men with advanced prostate cancer and found that 12% demonstrated BRCA2 loss and that 8% harbored ATM loss.[28] Furthermore, up to 19.3% of CRPCs demonstrate aberrations in BRCA1, BRCA2, or ATM; these events become more frequent as the disease progresses from hormone-sensitive to castration-resistant.[29] Together these data suggest that BRCAness is a reasonably frequent event in patients with advanced prostate cancer, which makes PARP inhibition an attractive target in this disease.

Synthetic lethality

The concept of promoting the killing of cancer cells by simultaneously blocking SSB repair using PARP inhibition in cells that lack efficient DSB repair is called “synthetic lethality.” In this scenario, tumor cells may harbor either germline or somatically acquired homozygous inactivation of HR. Germline defects (when present) typically affect only one allele in normal cells, and therefore normal tissues retain HR function. This difference between the DNA repair capacity of normal and cancer cells can be leveraged to produce selective cell killing of tumor cells by PARP inhibitors. Treatment of patients with PARP inhibitors will then block normal SSB repair in all cells, and these SSBs are subsequently converted to DSBs by DNA replication. In normal cells, HR restores the genome and allows survival, but in DRD cancer cells, DSBs persist, inducing cellular death selectively in the tumor cell population (see Figure 2).

Ample data indicate that PARP inhibitors possess antitumor activity within diverse patient populations, particularly those with BRCA1 or BRCA2 mutations.[14] One of the first studies to validate the concept of clinical benefit in patients with BRCA mutations was a phase I trial that looked at pharmacokinetic and pharmacodynamic aspects of olaparib treatment.[24] In this study, 60 patients with solid tumors were treated with various doses of olaparib (10 mg daily to 600 mg twice daily) to determine maximum tolerated dose (MTD). The study population was intentionally enriched for BRCA mutation carriers, and 22 patients of the cohort harbored BRCA1 or BRCA2 mutations. Objective tumor activity was observed in the mutation carrier population in patients with breast, ovarian, and prostate cancers. Three patients with advanced prostate cancer were included in this study cohort; the one with a BRCA2 mutation had a greater than 50% response in prostate-specific antigen (PSA) level, resolution of bone metastases, and an extended treatment course. This study suggested that there was a benefit of olaparib therapy in BRCA mutation carriers and the potential for benefit in prostate cancer patients. Further validation of olaparib efficacy in patients with BRCA mutations came from parallel proof-of-concept studies demonstrating the activity of this agent in women with breast and ovarian cancers and BRCA1 or BRCA2 mutations.[30,31] These data ultimately led to US Food and Drug Administration (FDA) approval of olaparib for women with a BRCA mutation and metastatic ovarian cancer after chemotherapy.
Additional data that demonstrate a similar spectrum of activity are available for other PARP inhibitors. Phase I data on the safety and pharmacodynamics of single-agent veliparib have been reported as an abstract,[32] and additional studies of veliparib in combination with mitomycin,[33] irinotecan,[34] and other agents have been reported.[35] VanderWeele et al published a case report of a patient with metastatic CRPC and BRCA2 mutation who had a sustained complete response to veliparib and carboplatin/gemcitabine.[36] It seems likely that many of the available PARP inhibitors may have overlapping activities, and further data will be needed to clarify which agent to use in which tumor type and the relative toxicities of each agent.

emozolomide and veliparib in metastatic CRPC

Compelling data implicate PARP1 in the mediation of DNA repair responses to alkylating agents,[37] cellular survival in BRCA-deficient cells,[24,38] and androgen receptor–mediated prostate cancer cellular proliferation.[9,39] Furthermore, data suggest that prostate cancers that harbor the TMPRSS2:ERG fusion (present in up to 50% of prostate cancers) may be more sensitive to PARP inhibition.[13] Therefore, Hussain et al carried out a single-arm pilot study to assess the safety and efficacy of veliparib with the alkylator temozolomide (TMZ) in patients with metastatic CRPC following docetaxel therapy.[40] In this study, patients with a PSA level of ≥ 2 ng/mL were treated with veliparib, 40 mg twice daily, on days 1 to 7 and TMZ, 150 to 200 mg/m2, on days 1 to 5 on a 28-day cycle, based on tolerance data from a phase I study (ClinicalTrials.gov identifier: NCT00526617). The primary endpoint was PSA response rate (30% decline). Of the 25 patients who were evaluable for response, 2 had a confirmed response, 13 had stable PSA, and 10 had progression. The most frequent toxicities were thrombocytopenia, anemia, fatigue, neutropenia, nausea, and constipation. The investigators did assess frequency of TMPRSS2:ERG fusion but found it in only one of eight evaluable patients. Although this patient had stable disease, no conclusions could be drawn regarding the contribution of the fusion product to veliparib sensitivity. Overall, while the combination was considered tolerable, it had only modest activity. No preselection was done in the study, and because BRCAness exists in 20% of patients, it is perhaps not surprising that activity was modest. The lower dose of PARP inhibitor and the lack of established benefit for TMZ may also have contributed to less than robust clinical activity for this combination. Given the emerging molecular data, it seems that future studies will be more likely to identify activity if done in preselected patient populations.

TOPARP

The Trial of PARP Inhibition in Prostate Cancer (TOPARP-A) sought to determine whether patients with prostate cancers with molecularly identified defects in DNA repair benefited from full-dose olaparib therapy.[25] In this phase II study, 50 men with CRPC underwent biopsy of metastatic disease and targeted next-generation sequencing, exome and transcriptome analysis, and digital polymerase chain reaction. The primary endpoint was response rate (either objective response or reduction of 50% in PSA level or reduction in circulating tumor cells). All had previously received docetaxel, and most had been treated with abiraterone or enzalutamide (98%) and cabazitaxel (58%). Patients were grouped according to the presence or absence of a homozygous deletion of or deleterious mutation in DNA damage response genes, which predict sensitivity to PARP inhibition. Overall, 16 of 49 evaluable patients (33%) were biomarker positive (indicative of homozygous deleterious changes in BRCA1/2, ATM, Fanconi anemia genes, or CHEK2). Of these, five patients had germline and somatic events (three patients with germline BRCA2 and three patients with germline ATM deletions or mutations). Of the 16 patients with deleterious changes in DNA repair genes, 14 (88%) responded to olaparib. The median overall survival for patients with biomarker-positive DRD tumors who received olaparib was 13.8 months, compared with 7.5 months for those with biomarker-negative tumors (P = .05). Interestingly, two biomarker-negative patients also met criteria for response to olaparib. Although one was a longer-term responder still on therapy at the time of publication, this particular patient did harbor monoallelic deletions of both BRCA2 and PALB2 that did not meet criteria for the prespecified biomarker-positive category but that may have contributed to tumor sensitivity. Toxicity was as expected, with patients displaying grade 3 or 4 anemia (10/50), fatigue (6/50), leukopenia (3/50), thrombocytopenia (2/50), and neutropenia (2/50). These results illustrate the feasibility of using molecular profiling to identify prostate cancers that display molecular features suggestive of sensitivity to PARP inhibition (BRCAness).

NCI 9012

ETS gene fusions—which result from gene rearrangement and juxtaposition of an androgen-responsive gene, such as TMPRSS2, to an ETS transcription factor gene, such as ERG or ETV1—occur in 50% to 60% of prostate cancers.[41,42] ETS transcription factors may also physically interact with PARP1, and PARP1 activity may be required for ETS-mediated invasion, transcription, and metastasis.[13] Androgen receptor–mediated transcription may also promote DNA DSBs and requires PARP activity for efficient repair.[43-45] Therefore, therapeutic targeting of androgen receptor signaling and PARP1 activity using abiraterone and veliparib is an attractive strategy in the management of metastatic prostate cancer.

A randomized phase II clinical trial in patients with metastatic CRPC was recently completed; it examined whether ETS fusion is a biomarker of response to abiraterone or abiraterone plus veliparib. In this study, 148 patients with metastatic CRPC underwent biopsy followed by assessment of ETS fusion status and then random assignment to either abiraterone alone or abiraterone plus veliparib. The primary endpoint was confirmed PSA response in patients receiving either abiraterone alone or combination therapy, stratified by ETS status. Secondary endpoints included safety, objective response rate, progression-free survival, and whether DNA repair gene deficiency (homozygous deletions of or deleterious mutations in: BRCA1, BRCA2, ATM, FANCA, PALB2, RAD51B, RAD51C) predicts response. This trial has now completed enrollment, and preliminary results will be presented at the American Society of Clinical Oncology 2016 Annual Meeting. Although final results are pending, the study does illustrate the feasibility of a large-scale metastatic tissue–based, biomarker-driven trial involving PARP inhibition in patients with metastatic CRPC. This study will also begin to ascertain the role of ETS fusions in determining response to PARP inhibitor therapy and will further explore the contribution of DRD to patient outcomes in those treated with standard therapy (abiraterone arm) and those treated with PARP inhibition (abiraterone plus veliparib arm).

Future studies

Given the data from the studies discussed previously and the enthusiasm for molecularly targeted trials in oncology, there is interest in further testing of PARP inhibition in prostate cancer patients. Multiple trials have recently been completed, are actively enrolling, or are nearing activation within this space (see Table, ClinicalTrials.gov).

Olaparib. Olaparib is the agent that is farthest along in clinical development and has an FDA indication in ovarian cancer. Olaparib also has the most active or pending studies in prostate cancer patients. TOPARP continues to enroll patients with metastatic CRPC, with a target accrual of 98 patients (ClinicalTrials.gov identifier: NCT01682772). There is a randomized double-blind, placebo-controlled phase II study of abiraterone plus olaparib or placebo for patients with metastatic CRPC who received prior docetaxel therapy (ClinicalTrials.gov identifier: NCT01972217). This trial, which is similar to the NCI 9012 study, has completed enrollment, but results are pending. Another trial is examining the biologic effect of olaparib on prostate cancer specimens when given alone or in combination with degarelix prior to prostatectomy (ClinicalTrials.gov identifier: NCT02324998). Furthermore, there is an open-label phase II study to assess the efficacy and safety of olaparib in patients with BRCA1 or BRCA2 mutations (regardless of tumor type), which is ongoing but no longer enrolling patients (ClinicalTrials.gov identifier: NCT01078662).

Veliparib. NCI 9012 (discussed previously) will help determine whether veliparib has potential therapeutic activity in metastatic CRPC and may identify molecularly determined subsets of disease (ie, ETS fusion–positive, DRD-positive) that might be expected to show the most benefit. The results of this study may help determine whether additional studies of this agent within the prostate cancer space are warranted.

Niraparib. The Hoosier Cancer Research Network has a planned phase I study of the combination of enzalutamide and niraparib for patients with metastatic CRPC (ClinicalTrials.gov identifier: NCT02500901), which has not yet begun enrollment. The primary endpoint of this study will be determination of MTD and dose-limiting toxicity.

Talazoparib. Although no prostate cancer–specific trials using other PARP inhibitors are currently active, several trials for molecularly targeted patient populations or phase I trials for toxicity assessment in combination with chemotherapy are ongoing; these provide some information on prostate cancer populations, depending on the types of solid tumors enrolled. There is a phase I trial of talazoparib in combination with carboplatin and paclitaxel (ClinicalTrials.gov identifier: NCT02317874) and another for patients with solid tumors and hepatic and renal dysfunction (ClinicalTrials.gov identifier: NCT02567396).

Precision Targeting of the PARP Pathway in Prostate Cancer

PARP inhibitors are a promising therapeutic option for men with prostate cancer. There is good evidence that men with either germline or somatic mutations in DNA repair pathways can derive therapeutic benefit from inhibition of PARP1/2, which blocks repair of SSB, driving persistent DSBs that lead to cancer cell lethality. Preclinical data also suggest that PARP inhibition may produce benefits by targeting chromatin and gene transcription, which implies that clinical benefits may extend beyond patients with DRD tumors.[12] To continue to develop PARP inhibitors within the prostate cancer field, we will need to develop and refine a set of biomarkers for use in selecting the right patient populations for these agents and then incorporate these biomarkers into prospective studies. As part of a precision therapy strategy, PARP inhibitors will likely play an important role in the management of prostate cancer in the near future.

It is now feasible to comprehensively profile the mutational, epigenetic, and gene expression changes in men with prostate cancer, and we are beginning to use this information to guide treatment choices.[7] Unfortunately, the functional relevance of many of the molecular features uncovered in these profiles is not completely understood. DNA repair processes are complex and require many genes for efficient repair of various types of DNA damage. Most past and ongoing studies focused on patients with specific molecular features, such as BRCA1, BRCA2, ATM, FANCA, PALB2, RAD51B, and RAD51C mutations. While mutations of these genes are likely to affect sensitivity to PARP inhibitors, mutations in other DNA repair or transcription factor genes may as well, and identification of those genes could expand the patient population that could benefit from therapy. Determination of whether other genes are susceptible to PARP inhibitor therapy will require robust preclinical models with a wide selection of genetic changes that reflect human disease; such models can be used to determine whether additional mutations and epigenetic or gene expression changes also result in PARP inhibitor sensitivity. Given the potential infrequency of many of the individual mutations that might sensitize to PARP inhibitors, large-scale registries that catalog mutations and their responsiveness to therapies may be needed.

As we define the molecular features that suggest sensitivity to PARP inhibition, the challenge will then become understanding the best strategy for incorporating these targeted agents into our standard treatment algorithms. In the context of prostate cancer, PARP inhibitors could be considered in high-risk patient populations in an adjuvant manner, before or with androgen deprivation therapy (ADT) in patients with newly metastatic disease, or in the setting of castration-resistant disease before or after the many other therapeutic options. To date, most trials in the prostate cancer space have been in the castration-resistant setting, perhaps because mutations in DNA damage genes may become more common as the disease progresses.[25] Nonetheless, there is no reason to assume that patients who harbor mutations may not benefit earlier in the disease course. Adjuvant use of PARP inhibitors in those with high-risk or micrometastatic disease could conceivably render patients disease free. Similarly, the combination of ADT and PARP inhibitors in early metastatic disease may provoke prolonged progression-free intervals similar to the situation with early docetaxel therapy but with less toxicity.[4,5] In the context of castration-resistant disease, it is reasonable to hypothesize that the combination of PARP inhibitors with hormonal agents such as abiraterone or enzalutamide or with chemotherapies might act synergistically to promote disease control.

The trials to examine these questions may be more challenging to design and execute because patients with sensitizing molecular changes represent a limited subset of total patients with prostate cancer. This means that in order to identify the subset that will benefit, many will need to be screened.[25] Because most molecular analyses are done using biopsy tissue, screening and cost may be challenging factors. In addition, the natural history of patients with DNA damage pathway mutations may also be distinct from those without such mutations. It is conceivable that mutations in DNA damage response genes may modulate patient response to standard hormonal agents, chemotherapy, or radium because all three of these therapeutic modalities have the potential to induce DNA damage in prostate cancer cells. Given these caveats, it will be essential to design an efficient precision medicine clinical trial pipeline that can rapidly molecularly profile patient tumors, assign to a therapeutic intervention, and then assess the complex resulting data and analyze results according to molecular categories.

PARP inhibitors have the potential to be a promising addition to the therapeutic arsenal used to treat prostate cancer and other solid tumors that harbor the appropriate molecular features. The transition from a standard, one-size-fits-all approach to a targeted, precision medicine strategy in which an individual prostate cancer patient’s tumor biology will guide choice of therapy will require careful planning and thought. The inclusion of PARP-targeted therapies before, after, with, or in place of standard hormonal therapies and chemotherapies will need to be defined so as to maximize antitumor effect and patient survival. Hopefully, application of these novel combinations in those most likely to benefit will ultimately lead to longer and better lives for patients with prostate cancer.

Financial Disclosure:Dr. Hussain is the principal investigator for a clinical trial of veliparib through the Cancer Therapy Evaluation Program (for AbbVie), and is collaborating on a clinical trial of olaparib for AstraZeneca.

How an Ovarian Cancer Drug Came to Have ‘Breakthrough Therapy Designation’ for Prostate Cancer

With the emergence of precision medicine, clinicians can now take advantage of high-throughput tumor sequencing to identify driver mutations in individuals with cancer, with the goal of matching these with effective therapies. Since driver mutations can be shared across cancer types, precision medicine has also challenged the notion that cancer types, as defined by site of origin, are completely separate entities. One such example is the use of vemurafenib in multiple BRAF V600–mutant cancers. Another example is that of poly(adenosine diphosphate [ADP]–ribose) polymerase (PARP) inhibitors and prostate cancer. It is now recognized that DNA repair abnormalities, including and most notably BRCA2 mutations, are found frequently in the germline and as somatic mutations in the tumors in men with metastatic prostate cancer. Moreover, recent studies have demonstrated promising activity for olaparib—a drug approved for use in BRCA-mutated ovarian cancer—in men with castration-resistant disease and germline or somatic DNA repair abnormalities. This has led the US Food and Drug Administration to confer “breakthrough therapy designation” on olaparib, based on the strong belief that the drug will ultimately be approved for this indication.

What Questions Should Future Research on PARP Inhibitors for Prostate Cancer Focus on?

Many questions still remain unanswered. These include:

1) Given the pleiotropic effects of PARP inhibitors, which activities are the most critical and which PARP inhibitors are best for each disease/mutation scenario?

2) Have we identified the full gamut of DNA repair abnormalities that might respond to PARP inhibition?

3) Can we extend the spectrum of patients eligible for PARP inhibitors to those who are homologous recombination–proficient, by combining PARP inhibitors with therapies such as alkylating agents or antiangiogenic agents like cediranib?

4) Can we identify patients early on in their disease course in whom PARP inhibition may contribute to a curative strategy?

Scientists at the Weizmann Institute may have found the cure for prostate cancer, at least if it is caught in its early stages – via a drug that doctors inject into cancerous cells and treat with infrared laser illumination.

Using a therapy lasting 90 minutes, the drug, called Tookad Soluble, targets and destroys cancerous prostate cells, studies show, allowing patients to check out of the hospital the same day without the debilitating effects of chemical or radiation therapy or the invasive surgery that is usually used to treat this disease.

The drug has been tested in Europe and in several Latin American countries, and is being marketed by Steba Biotech, an Israeli biotech start-up with R&D facilities in Ness Ziona. The drug and its accompanying therapy were developed in the lab of Weizmann Institute professors Yoram Salomon of the Biological Regulation Department and Avigdor Scherz of the Plant and Environmental Sciences Department.

Based on principles of photosynthesis, the drug uses infrared illumination to activate elements that choke off cancer cells, but spares the healthy ones.

The therapy was recently approved for marketing in Mexico, after a two-year Phase III clinical trial in which 80 patients from Mexico, Peru and Panama who suffered from early-stage prostate cancer were treated with the Tookad system. Two years after treatment, over 80% of the study’s subjects remained cancer-free.

A similar study being undertaken in Europe showed similar results, Steba Biotech said, and the company had submitted a marketing authorization application to the European Medicine Agency for authorization of Tookad as a treatment of localized prostate cancer.

The approved therapy was developed by Salomon and Scherz using a clever twist on photosynthesis called photodynamic therapy, in which elements are activated when they are exposed to a specific wavelength of light.

Tookad was first synthesized in Scherz’s lab from bacteriochlorophyll, the photosynthetic pigment of a type of aquatic bacteria that draw their energy supply from sunlight. Photosynthesis style, the infrared light activates Tookad (via thin optic fibers that are inserted into the cancerous prostatic tissue) which consists of oxygen and nitric oxide radicals that initiate occlusion and destruction of the tumor blood vessels.

These elements are toxic to the cancer cells and once the Tookad formula is activated, they invade the cancer cells, preventing them from absorbing oxygen and choking them until they are dead. The Tookad solution, having done its job, is supposed to then be ejected from the body, with no lingering consequences – and no more cancer.

With the drug approved for prostate cancer – and able to reach cancerous cells that are deep within the body via a minimally invasive procedure – Steba believes it may be able to treat other forms of cancer. In fact, the company said, it is also pursuing early stage studies of Tookad in esophageal cancer, urothelial carcinoma, advanced prostate cancer, renal carcinoma, and triple negative breast cancer in collaboration with Memorial Sloan Kettering Cancer Center, the Weizmann Institute, and Oxford University.

“The use of near-infrared illumination, together with the rapid clearance of the drug from the body and the unique non-thermal mechanism of action, makes it possible to safely treat large, deeply embedded cancerous tissue using a minimally invasive procedure,” according to Steba.

The Weizmann Institute has been working with Steba researchers for some 20 years to develop Tookad, said Amir Naiberg, CEO of the Yeda Research and Development Company, the Weizmann Institute’s technology transfer arm and the licensor of the therapy. “The commitment made by the shareholders of Steba and their personal relationship and effective collaboration with Weizmann Institute scientists and Yeda have enabled this tremendous accomplishment.”

“We are excited to bring a unique and innovative solution to physicians and patients for the management of low-risk prostate cancer in Mexico and subsequently to other Latin American countries,” said Raphael Harari, chief executive officer of Steba Biotech. “This approval is recognition of the tremendous effort deployed over the years by the scientists of Steba Biotech and the Weizmann Institute to develop a therapy that can control effectively low-risk prostate cancer while preserving patients’ quality of life.”

Abstract: Cabazitaxel provided a survival advantage compared with mitoxantrone in patients with castration-resistant prostate cancer refractory to docetaxel. Grade 3 to 4 (G3–4) neutropenia and febrile neutropenia were relatively frequent in the registrative XRP6258 Plus Prednisone Compared to Mitoxantrone Plus Prednisone in Hormone Refractory Metastatic Prostate Cancer (TROPIC) trial, but their incidence was lower in the Expanded Access Program (EAP). Although cumulative doses of docetaxel are associated with neuropathy, the effect of cumulative doses of cabazitaxel is unknown. In this retrospective review of prospectively collected data, the authors assessed “per cycle” incidence and predictors of toxicity in the Italian cohort of the EAP, with a focus on the effect of cumulative doses of cabazitaxel.

The study population consisted of 218 Italian patients enrolled in the cabazitaxel EAP. The influence of selected variables on the most relevant adverse events identified was assessed using a Generalized Estimating Equations model at univariate and multivariate analysis.

Among the toxicities assessed, the authors did not identify any that appeared to be associated with a higher number of cabazitaxel cycles delivered. Prior cumulative dose was associated with reduced G3 to 4 neutropenia and anemia. The apparent protective effect associated with higher doses of cabazitaxel is likely to be affected by early dose reduction and early toxicity-related treatment discontinuation. Because this analysis is limited by its retrospective design, prospective trials are required to assess the optimal duration of cabazitaxel treatment.

Several agents provide a survival advantage and symptom palliation in patients with docetaxel-refractory, metastatic castration-resistant prostate cancer (CRPC).1,2 These agents include cabazitaxel, enzalutamide, abiraterone, and radium 223.1,2 Presently, the treatment choice is influenced by several factors, including physician’s and patient’s preference, drug availability, reimbursement policies, performance status, organ function, as well as expected toxicity profile, but comparative efficacy data are lacking. Similarly to other taxane agents, cabazitaxel is frequently associated with bone marrow toxicity. In the XRP6258 Plus Prednisone Compared to Mitoxantrone Plus Prednisone in Hormone Refractory Metastatic Prostate Cancer (TROPIC) trial,3 grade (G) 3–4 neutropenia and febrile neutropenia were reported in 82% and 8% of patients treated with cabazitaxel, respectively, whereas these adverse events were respectively reported in 33.9% and 5% of the Italian patients enrolled in the Expanded Access Program (EAP).4 Conversely, G3 to 4 neuropathy was a rare event both in the TROPIC and in the EAP study.3–5 To further analyze the safety profile of cabazitaxel, we retrospectively reviewed prospectively collected data about the most common toxicities reported in the Italian cohort of the EAP. “Per cycle,” rather than “per patient” incidence was computed, and an explorative analysis was performed to investigate potential predictors of toxicity. In view of the risk of cumulative toxicity (neuropathy) associated with docetaxel,6 the effect of prior cumulative dose of cabazitaxel was investigated in a multivariable model along with other potential predictive factors.

Treatment

At the time of the analysis, 1494 cycles had been administered to 218 patients included in the entire cohort, whereas a total of 553 cycles had been administered to 61 patients with a body surface area >2 sqm. Patients were administered a median of 6.0 (interquartile range: IR, 4–10) cycles. The median dose delivered was 24.00 mg/sqm (IR: 22.3–24.7). Each patient received a median cumulative dose of 149.9 mg/sqm (IR: 92.8–232.2). Sixty-four patients (29.6%) received at least 10 cycles (Table 2). Primary G-CSF prophylaxis was administered in 87 patients (39.9%), whereas G-CSF secondary prophylaxis was administered in 76 patients (34.8%). Therapy was delayed in 274 cycles, which was because of cabazitaxel toxicity only in 65 (23.7%) of these. Dose was reduced 52 times (Table 2), and in 45 cases dose reduction was because of cabazitaxel adverse events. In the safety population, the main reason for treatment discontinuation was disease progression (43.1%), followed by adverse event (24.5%) and physician’s decision (18.5%). Of note, in the subgroup of 64 patients receiving at least 10 cycles, 51.6% discontinued cabazitaxel because of investigator’s decision, and only 1 patient (1.6%) discontinued for toxicity (Table 3).

Table 2Image Tools

Table 3Image Tools

Safety

Overall incidence of toxicity per cycle is detailed in Table 4. Main G3 to 4 hematologic toxicities were neutropenia and anemia. The “per cycle” incidence rate of G3 to 4 neutropenia was 8.7%, whereas febrile neutropenia occurred only in 0.9% of all cycles and it was an early event, occurring during the first 3 cycles only (Figure 1). Main non hematologic toxicities were G2 asthenia/fatigue and G2 diarrhea, occurring in 3.7% and 0.8% of cycles, whereas G3 to 4 asthenia/fatigue and G3 to 4 diarrhea occurred in 1.8% and 0.4% of cycles. Four adverse events had a per cycle incidence >1% and were selected for univariate (Tables 5 and 6) and multivariate (Table 7) analysis GEE logistic regression analysis. Febrile neutropenia was also assessed because of its clinical relevance. Multivariate logistic regression analysis showed a significant reduction of the odds of having G3 to 4 neutropenia (−10%), febrile neutropenia (−48%) and anemia (−7%), per 10 mg/m2 increase of total prior dose of cabazitaxel. A body surface area >2 m2 was associated with increased odds of having G3 to 4 neutropenia (OR: 2.58; 95% CI = 1.50–4.43; P < 0.01), but decreased odds of having G3 to 4 anemia (OR: 0.10; 95% CI = 0.02–0.52; P < 0.01). Age as a continuous variable was not associated to an increased rate of any of the adverse events analyzed. Of note, higher previous dose of docetaxel appeared to be associated with a slightly, but statistically significant decreased odds of having G 3–4 anemia (OR: 0.859; 95% CI = 0.73−1.00; P = 0.06), G3 to 4 neutropenia (OR: 0.95; 95% CI = 0.91–0.99; P = 0.03), and G2 and G3 to 4 fatigue/asthenia (OR: 0.90; 95% CI = 0.84–0.96; P < 0.01). Twelve patients died within 30 days since last cabazitaxel treatment for causes judged to be unrelated to cabazitaxel by the local investigators. Three patients died as a result of treatment-emergent adverse events possibly related to cabazitaxel treatment. Of these 3 patients, 1 patient died after 1 cycle because of respiratory and renal failure, 1 patient died after 2 cycles because of respiratory failure and the third patient died after 3 cycles because of pancytopenia and hepatic failure.

Table 4Image Tools

Figure 1Image Tools

Table 5Image Tools

Table 6Image Tools

Table 7Image Tools

DISCUSSION

In a cohort of 746 patients enrolled throughout Europe in the cabazitaxel EAP, G3 to 4 neutropenia, febrile neutropenia and G3 to 4 diarrhea occurred in 17%, 5.4% and 2.8% of patients, respectively.5The discrepancy of these results with those obtained in the TROPIC trial has been explained by study differences in patient characteristics, frequency of hematologic assessment, as well as proactive management of adverse events of cabazitaxel.5 Dose reductions were also more frequent in the EAP compared with the TROPIC trial (17.4% versus 12%) and may also have affected the safety profile.4,5 Furthermore, in the EAP versus the TROPIC trial, 1% versus 2% of patients died as a result of neutropenia, respectively. In our study cohort, only 3 deaths (≈1.3%) possibly related to cabazitaxel treatment were reported, whereas 12 patients died within 30 days since the last cabazitaxel dose for reasons, which were definitely judged to be unrelated to cabazitaxel by the local investigator. Treatment delay, which was reported in 274 cycles, was because of cabazitaxel toxicity approximately only in one-fourth of cases and to “other causes” in 180 cases. This finding may be related to the influence of logistic reasons (eg, waiting list) or patient’s compliance as a common cause of treatment delay. Dose reduction, which was reported in 52 cases, was mainly because of cabazitaxel adverse events. Ongoing phase III trials are assessing whether lower doses of cabazitaxel are equally effective and better tolerated than higher doses.7 In the analysis of our study cohort, the dose of 25 versus 20 mg/m^2 was associated with increased risk of G3 to 4 neutropenia (OR = 1.8; CI = 1.0–3.55; P = 0.049) in the multivariable model, but this result is likely to be confounded by patients who received the 25 mg/m^2 dose and then permanently interrupted treatment for toxicity. Differently from the results obtained in other series,5,8 we have not found the use of G-CSF to be associated with decreased incidence of G3 to 4 neutropenia, possibly because frailer patients are both more likely to experience G 3–4 neutropenia and to receive G-CSF prophylaxis. Similarly to the results obtained in the work by Heidenreich et al,8 we found that prior cumulative dose of docetaxel was associated with lower odds of G3 to 4 neutropenia. Reintroduction of docetaxel was reported to be a feasible option in selected patients, although docetaxel rechallenge is not supported by randomized-controlled trials.9,10 A favorable association of prior cumulative dose of docetaxel with G3 to 4 anemia and G 2–4 asthenia/fatigue was also reported, along with an overall low “per cycle incidence” of febrile neutropenia and G3 to 4 diarrhea and neutropenia. These toxicities do not recur throughout the course of the treatment in most of the cases. Higher prior cumulative dose of cabazitaxel was associated with lower risk of G3 to 4 neutropenia and febrile neutropenia, and the majority of G3 to 4 events of bone marrow toxicity occurred during the first 5 cycles. Heidenreich et al5 compared toxicities associated with first versus subsequent doses and reported higher odds of severe neutropenia at the first cycle versus subsequent cycles. This result is consistent with existing data.11 In our work, we found no evidence of cumulative toxicity for any of the adverse events considered in a multivariable model assessing their association with prior cumulative dose of cabazitaxel. In this regard, it is noteworthy that of the 64 patients receiving at least 10 cycles, only 1 (1.5%) had to interrupt treatment because of toxicity and approximately 50% (33 patients, 51.6%) suspended treatment because of investigator’s decision. Although continuation of docetaxel after 10 cycles does not appear to yield any benefit,12 the optimal duration of cabazitaxel treatment in nonprogressive patients is unknown.

No studies have been specifically conducted to assess the additional benefit associated with continuation of cabazitaxel treatment beyond 10 cycles. Nevertheless, the risk of rapidly progressive disease following cabazitaxel interruption must be carefully considered and discussed with the patient, especially in those with high disease burden who may experience clinical deterioration and be unable to resume systemic therapy.13 We also reported that patients with a body surface area greater than 2 m2 showed an OR of 2.58 for G3 to 4 neutropenia, but an OR of 0.1 for G3 to 4 anemia. We are unable to provide an explanation for this finding at the present time.

Our analysis has a number of limitations, including its retrospective nature, the arbitrary selection of the variables included in the multivariable model, the lack of sample size calculation, as well as the lack of assessment of peripheral neuropathy, which is a clinically relevant adverse event in patients receiving chemotherapy after first-line docetaxel6,14. Furthermore, the number of patients receiving >10 cycles was small and no patient received more than 17 cycles. In this regard, it must be noted that a report of 4 patients with CRPC cancer treated with >15 cycles of cabazitaxel found that peripheral neuropathy was the only clinically significant toxicity associated with cumulative doses.15 There is no established clinical variable predictive of cabazitaxel efficacy in the postdocetaxel setting, although preliminary evidence by our work group suggest that cabazitaxel could be more effective than novel hormonal agents in a number of clinical settings, which include patients with brain metastases,16 high Gleason score at diagnosis,17 and primary refractoriness to docetaxel.18 Similarly to other antineoplastic agents (eg, sunitinib19), cabazitaxel may also be more effective in patients showing greater treatment-related toxicity. A recent post-hoc analysis of the TROPIC trial suggested that treatment outcomes, in terms of Overall Survival, Progression Free Survival, and Prostate Specific Antigen response, were improved in patients developing G3 to 4 neutropenia.20 Our analysis confirms that the safety profile of cabazitaxel compares favorably with that of docetaxel, which was associated with G3 to 4 diarrhea, nail changes, and peripheral neuropathy in approximately 30% of the patients.21 Among the toxicities assessed, we did not identify any that appeared to be dependent on the cumulative dose of cabazitaxel priorly administered. As this finding is likely to be influenced by early dose reduction and early toxicity-related treatment discontinuation, it must be confirmed by prospective larger trials in patients with metastatic castration resistant prostate cancer.

University of Liverpool Scientists Report New Urine Test To Detect Potential Biomarkers of Prostate Cancer

A research team from the University of Liverpool has reached an important milestone towards creating a urine diagnostic test for prostate cancer that could mean that invasive diagnostic procedures that men currently undergo eventually become a thing of the past.

‘The use of a gas chromatography (GC)-sensor system combined with advanced statistical methods towards the diagnosis of urological malignancies’, published today in the Journal of Breath Research, describes a diagnostic test using a special tool to ‘smell’ the cancer in men’s urine.

Working in collaboration with the University of the West of England’s (UWE Bristol) Urological Institute team at Southmead Hospital and Bristol Royal Infirmary, the pilot study included 155 men presenting to urology clinics. Of this group, 58 were diagnosed with prostate cancer, 24 with bladder cancer and 73 with haematuria or poor stream without cancer. The results of the pilot study using the GC sensor system indicate that it is able to successfully identify different patterns of volatile compounds that allow classification of urine samples from patients with urological cancers.

Urgent need for earlier diagnosis

Professor Chris Probert from the University of Liverpool’s Institute of Translational Medicine began work on this project with UWE Bristol when he was working in Bristol as a gastroenterologist with clinical and research interest in inflammatory bowel disease.

The research team used a gas chromatography sensor system called Odoreader that was developed by a team led by Professor Probert and Professor Norman Ratcliffe at UWE Bristol. The test involves inserting urine samples into the Odoreader that are then measured using algorithms developed by the research team at the University of Liverpool and UWE Bristol.

Professor Probert said: “There is an urgent need to identify these cancers at an earlier stage when they are more treatable as the earlier a person is diagnosed the better. After further sample testing the next step is to take this technology and put it into a user friendly format. With help from industry partners we will be able to further develop the Odoreader, which will enable it to be used where it is needed most; at a patient’s bedside, in a doctor’s surgery, in a clinic or Walk In Centre, providing fast, inexpensive, accurate results.”

Like an electronic nose

Professor Norman Ratcliffe said, “There is currently no accurate test for prostate cancer, the vagaries of the PSA test indicators can sometimes result in unnecessary biopsies, resulting in psychological toll, risk of infection from the procedure and even sometimes missing cancer cases. Our aim is to create a test that avoids this procedure at initial diagnosis by detecting cancer in a non-invasive way by smelling the disease in men’s urine. A few years ago we did similar work to detect bladder cancer following a discovery that dogs could sniff out cancer. We have been using the Odoreader, which is like an electronic nose to sense the cancer.”

“The Odoreader has a 30 metre column that enables the compounds in the urine to travel through at different rates thus breaking the sample into a readable format. This is then translated into an algorithm enabling detection of cancer by reading the patterns presented. The positioning of the prostate gland which is very close to the bladder gives the urine profile a different algorithm if the man has cancer.”

Mr Raj Prasad, Consultant Urologist at Southmead Hospital, North Bristol NHS Trust, said: “If this test succeeds at full medical trial it will revolutionise diagnostics. Even with detailed template biopsies there is a risk that we may fail to detect prostate cancer in some cases. Currently indicators such as diagnosed prostatomegaly (enlarged prostate) and unusually high PSA levels can lead to recommendations for biopsy if there is a concern that cancer may be prevalent. An accurate urine test would mean that many men who currently undergo prostate biopsy may not need to do so.”

A link to the research article in the Journal of Breath Research is given below:

Abstract

Prostate cancer is one of the most common cancers. Serum prostate-specific antigen (PSA) is used to aid the selection of men undergoing biopsies. Its use remains controversial. We propose a GC-sensor algorithm system for classifying urine samples from patients with urological symptoms. This pilot study includes 155 men presenting to urology clinics, 58 were diagnosed with prostate cancer, 24 with bladder cancer and 73 with haematuria and or poor stream, without cancer. Principal component analysis (PCA) was applied to assess the discrimination achieved, while linear discriminant analysis (LDA) and support vector machine (SVM) were used as statistical models for sample classification. Leave-one-out cross-validation (LOOCV), repeated 10-fold cross-validation (10FoldCV), repeated double cross-validation (DoubleCV) and Monte Carlo permutations were applied to assess performance.

Significant separation was found between prostate cancer and control samples, bladder cancer and controls and between bladder and prostate cancer samples. For prostate cancer diagnosis, the GC/SVM system classified samples with 95% sensitivity and 96% specificity after LOOCV. For bladder cancer diagnosis, the SVM reported 96% sensitivity and 100% specificity after LOOCV, while the DoubleCV reported 87% sensitivity and 99% specificity, with SVM showing 78% and 98% sensitivity between prostate and bladder cancer samples. Evaluation of the results of the Monte Carlo permutation of class labels obtained chance-like accuracy values around 50% suggesting the observed results for bladder cancer and prostate cancer detection are not due to over fitting.

The results of the pilot study presented here indicate that the GC system is able to successfully identify patterns that allow classification of urine samples from patients with urological cancers. An accurate diagnosis based on urine samples would reduce the number of negative prostate biopsies performed, and the frequency of surveillance cystoscopy for bladder cancer patients. Larger cohort studies are planned to investigate the potential of this system. Future work may lead to non-invasive breath analyses for diagnosing urological conditions.

Other articles in this Open Access Journal related to Cancer Detection Systems include:

Prostate cancer screening using prostate-specific antigen (PSA) is highly controversial. In this Q & A, Guest Editors for BMC Medicine’s ‘Spotlight on Prostate Cancer’ article collection, Sigrid Carlsson and Andrew Vickers, invite some of the world’s key opinion leaders to discuss who, and when, to screen for prostate cancer. In response to the points of view from the invited experts, the Guest Editors summarize the experts’ views and give their own personal opinions on PSA screening.

Screening for prostate cancer with prostate-specific antigen (PSA) is controversial. Screening is currently transitioning from being an all-or-nothing-question, to finding new ways of individualized testing. However, consensus remains to be reached within guideline groups and worldwide experts regarding who – and when – to screen, if at all. In this Q & A, we invite seven of the world’s key opinion leaders in the field, both proponents and skeptics, to elaborate on what they believe the current screening policy should be. The authors have all published widely on PSA, and comprise a wide variety of experience in areas such as urology, epidemiology, evidence-based medicine, and medical decision-making.

Currently, only one guideline group, the United States Preventive Services Task Force (USPSTF), recommends against screening for all men [1]. Most other guideline groups recommend shared decision-making, involving a discussion of the pros and cons of screening [2]. To aid in decision-making, some propose using a risk-stratified approach taking into account multiple factors along with a PSA measurement [3]. However, the specifics of such an approach are a subject of debate; for instance, the appropriate age limits of screening remain to be defined. Randomized screening trials, including the European Randomized Study of Screening for Prostate Cancer (ERSPC) and the Göteborg trial [4, 5] have provided evidence that regular PSA-screening can reduce prostate cancer mortality by 21–44 % at 13–14 years of follow-up; the age groups studied in these trials were 55–69 and 50–64 years, respectively. Thus, the question remains regarding the screening of men outside this age range. There is a growing body of evidence on the benefits of commencing screening in the mid-40s. While the American Urological Association (AUA) bases its recommendation on the 55–69 age group based on the ERSPC results [6], the European Urological Association recommends a baseline PSA be obtained at 40–45 years of age [7].

Our personal view is that PSA screening should indeed involve shared decision-making, but we believe the focus should primarily be on behavior, rather than preference. For this purpose, we have published a decision-support tool called the ‘Simple Schema’ [8], which acknowledges that the majority of harms of screening result from unnecessary treatment of low-risk disease and therefore focuses on the importance of active surveillance as the appropriate, evidence-based management strategy for low-risk cancer [9–11]. We further believe that PSA screening should be a risk-stratified approach aimed at detecting lethal prostate cancer. This is based on evidence that only a small proportion of men with moderately elevated PSA have aggressive disease [5] and that overdiagnosis is strongly influenced by age and PSA levels. For instance, we have shown that almost half of the excess incidence of cancer associated with PSA testing occurs in men over 70 [12] – a group in which screening is likely of little, if any, benefit [5, 13] – and that the effects of screening men in their 60s is highly dependent on their PSA level, with an excellent ratio of harms to benefits in patients with PSA ≥2 ng/mL but zero benefit in patients with a lower PSA [14]. Therefore, the current guidelines in place at Memorial Sloan Kettering Cancer Center restrict screening in men over 60 to those with above average PSAs and dramatically restrict screening in men over 70 to a small number of men with exceptional health and high PSA [15]. Additionally, biopsy is recommended only after a repeat PSA and further work-up, and the frequency of screening is stratified depending on baseline PSA, which has been shown to be a very strong predictor of long-term prostate cancer metastasis and death [16–18].

Who and when to screen, and not to screen, for prostate cancer: the proponents’ view

M Leapman & P Carroll: Prostate cancer is a highly prevalent disease that exhibits considerable variation in clinical behavior; some men will enjoy long lives with low-grade disease without treatment, while a significant, but smaller number may succumb to a devastating metastatic burden. Therefore, the current challenge faced in the early management of prostate cancer is to identify and treat disease in men likely to obtain benefit while sparing unnecessary detection and, most importantly, treatment in those who will not.

Compelling randomized evidence suggests that PSA screening of asymptomatic men is associated with a significant reduction in death from prostate cancer. The recent 13-year update of the ERSPC trial, a multi-centered study examining prostate cancer mortality among participants aged 50–74 receiving regular PSA screening compared with no routine screening, has demonstrated a relative risk of death from prostate cancer of 0.79 (95 % CI, 0.69–0.91, P = 0.001) in favor of screening men aged 55–69 years. Moreover, when adjusting for non-participation, the reduction in risk increased to 27 % (95 % CI, 0.61–0.88, P < 0.0007) [5]. Naturally, screening can only reduce mortality by effecting treatment of clinically significant disease, as was observed in the ERSPC experience, in which substantially more cancers were diagnosed and subsequently treated [19].

It is true that eliminating PSA screening will obviously decrease the number of ‘insignificant’ prostate cancer cases detected; however, this would come at the imprudent expense of ignoring disease in intermediate and high-risk patients who may stand to benefit substantially. Indeed, the rationale for treatment in appropriate patients is redoubled by randomized evidence suggesting improvement in survival and metastatic progression with timely treatment of threatening cancers [20]. However, an overall improvement in prostate cancer mortality is alone insufficient to justify expansive screening and treatment of all men if such a strategy will expose those harboring non-lethal tumors to a non-trivial risk of adverse quality of life outcomes; one should not screen if overdiagnosis is followed by overtreatment [21].

Ultimately, the landscape of early prostate cancer detection may not truly be cast in a monochromatic decision palette, namely to screen with PSA or not to screen. Rational practices including the screening of healthy men with a long life expectancy starting at age 45, cessation of screening in those with significant co-morbidity and those of advanced age, extending the interval of screening in most men (2–4 years), and discontinuing screening for those with low-risk profiles at certain ages alone would significantly improve the efficiency of early detection. In addition, nuanced strategies for prostate cancer detection and management represent an auspicious frontier. Although not validated in randomized trials of screening techniques, assays incorporating novel PSA isoforms – including the 4-Kallikrein panel [22] and the Prostate Health Index [23] – appear to add much needed specificity for the detection of high-grade (Gleason ≥7) prostate cancer at a diagnostic prostate biopsy, thereby potentially reducing the number of unnecessary biopsies performed. Advanced imaging modalities, including multi-parametric MRI, may also better refine the candidacy and yield of biopsy [24]. Among newly diagnosed patients, tumor-based risk stratification methods [25] and favorable long-term experiences with active surveillance [10] are poised to improve the confidence and quality with which incidental tumors are managed. Such measures, being currently clinically implemented, are highly promising means to cultivate better screening; these methods will highlight prostate cancer requiring attention while disregarding or proposing active surveillance of those that do not.

F Schröder: In my view, the time for population-based screening has not come and may never do so. The main reason for my pessimistic view on this issue is the high probability (of approximately 40 %) of diagnosing cancers which will not progress clinically, cause symptoms, or lead to death (overdiagnosis) [5]. While there are methods available to decrease overdiagnosis, such as the use of risk calculators and multiparametric MRI of the prostate, these have not been sufficiently validated in multicenter use to establish their accuracy in routine clinical practice. As a main contributor and former principal investigator of the ERSPC screening trial, I am delighted to see increasing worldwide acceptance of the recommendation of our group, including in US, European, and Russian guidelines to apply ‘shared decision-making’. We recommend the use of a procedure developed on the basis of ERSPC data, which is freely available on the website of the SIU [26].

A Vickers and S Carlsson [to M Leapman and P Carroll]: Could you please clarify what you think current policy should be and whether all men should be screened?

M Leapman & P Carroll: On the basis of randomized screening trials, PSA screening should be offered to healthy individuals without known risk factors for prostate cancer beginning at age 45 [5, 27]. No stark policy, however, will account for the complexities involved with screening and early diagnosis of prostate cancer. As a result, screening of asymptomatic men should be approached in the setting of a shared decision between a patient and physician cognizant of the individual’s age, health status, personal preferences, and risk factors including family history, race, prior PSA, and biopsy status. As noted above, the efficiency of screening can be improved and contemporary screening guidelines are incorporating refinements as suggested [28].

A Vickers and S Carlsson: You say that “discontinuing screening for those with low baseline risk profiles[would]significantly improve the efficiency of[PSA]screening”. Could you give a specific example of the profile of a man for whom screening should be discontinued?

M Leapman & P Carroll: Obviously, those in poor health or of advanced age do not benefit from early detection efforts. The optimal frequency of PSA testing has not been explicitly compared in a randomized fashion, but screening at 2–4 year intervals appears appropriate in low-risk patients based on PSA levels. Baseline PSA status may offer a valuable insight into a patient’s further risk for harboring or developing significant prostate cancer. Persuasive evidence from a Swedish population-based cohort examining PSA levels at age 60 suggests that men with levels <1 ng/mL possess a low (near zero) risk of prostate cancer death in extended follow-up [14, 17]. It would be reasonable to forego further screening of a 60-year-old without other known risk factors with baseline PSA <1 in the absence of cause.

in the state of Michigan, where nearly half of all patients diagnosed with low-risk disease are initially managed with active surveillance [30]

F Schröder: Weighing pros and cons of a medical procedure will always remain the responsibility of the person at risk. We, as doctors and urologists, have to help by addressing the questions arising after reading the first section of the decision aid, which is only one page.

Who and when to screen, and not to screen, for prostate cancer: the skeptics’ view

should PSA be avoided entirely in asymptomatic men, or should screening be restricted to certain subgroups and, if so, whom?

P Albertsen: Few cancers generate as much controversy as prostate cancer concerning screening, diagnosis, and treatment. From 1977 to 2005, the lifetime risk of prostate cancer diagnosis in the US increased from 7.3 % to 17 % [31, 32]. During this same period, the lifetime risk of dying from prostate cancer fell from 3.0 % to 2.4 %.

My views on prostate cancer screening and treatment have been shaped by my training in urology at Johns Hopkins and epidemiology and public health at the University of Wisconsin, as well as by my patients. One patient in particular had a powerful influence. He was referred for treatment of a localized prostate cancer and underwent successful surgery. His postoperative PSA was undetectable and all surgical margins were negative. I told him he was cured only to have him return 2 years later with widespread metastases. I treated him successfully with anti-androgen therapy for an additional 16 years. According to the Halsted model of cancer progression he should have been cured by surgery [33]. I had the original specimen re-cut to document negative margins and concluded that we had a poor understanding of the natural history of this disease and the efficacy of treatment.

To address this problem, I developed a Markov model of prostate cancer progression. With the assistance of Jack Wennberg’s research group, the model was published in JAMA [34]. At the time, it was severely criticized by the urologic community, but when reviewed today the model bears a remarkable resemblance to data recently published by the SPCG-4 [35]. The sensitivity analysis indicated that data concerning the natural history of this disease was most critical to estimating the relative value of intervention. This is why I gathered data on the natural history of this disease from patients enrolled in the Connecticut tumor registry and published them in JAMA in 1998 and 2005 [36, 37].

My training in epidemiology taught me to view healthcare delivery from a public health perspective. Screening programs were best assessed by addressing four key questions: (1) Is prostate cancer a suitable disease for screening? (2) Is treatment for prostate cancer effective? (3) Is PSA an effective screening test for this disease? (4) Does screening result in any harm?

Only recently have we begun to appreciate that a large number of men harbor indolent disease, as clearly demonstrated by data from the Finasteride Chemoprevention trial [39]. Many pathologists now question whether Gleason 3 + 3 tumors are sufficiently aggressive to cause morbidity [40]. Indeed, recent data from the ERSPC trials suggest that half of all screen-detected cancers are indolent [5].

The efficacy of treatment also poses problems. Most urologists and radiation therapists assume that surgery and radiation are curative. However, what do the data say? The SPCG-4 suggests that some men are cured by surgery, but many men are not [35]. Further, men with high-grade disease often die from prostate cancer despite surgery and that surgery is primarily palliative for men aged over 65. With regards to radiation, even less information concerning its efficacy is available to date.

How well does PSA perform as a screening tool? Unfortunately, many lethal tumors produce low amounts of PSA and are missed by screening studies. Additionally, prostate enlargement, prostatitis, and surgical manipulation can lead to a significant number of false positive values. Finally, screening can result in considerable morbidity as documented by the USPSTF report [1].

I have been an advocate of active surveillance for men with low-grade disease [9]. Data from the ProTECT trial should provide important new data concerning efficacy when published in the spring of 2016 [41]. Ideally, screening should only identify men destined to suffer from clinically significant disease and patients should only be offered treatments that yield substantial benefit.

D Ilic: Prostate cancer is a leading cancer affecting men worldwide [42]. Despite its high prevalence, the current evidence suggests that screening asymptomatic men for prostate cancer is not warranted [1, 43]. The most recent Cochrane systematic review identified five randomized controlled trials examining the effectiveness of screening [43]. A meta-analysis of data from those five trials determined no significant difference in prostate cancer mortality between men randomised to screening in comparison to those who were not (risk ratio (RR) = 1.00; 95 % CI, 0.86–1.17) [43].

Only two of the five randomized controlled trials included in the 2013 systematic review were determined to methodologically present a low-risk of bias [43]. The point of interest lies in the differing results and conclusions offered by two studies: the ERSPC and the Prostate, Lung, Colorectal and Ovarian (PLCO) cancer screening trial [27, 44].

In 2014, the ERSPC published 13-year follow-up data, reporting a 21 % reduction in the risk of prostate cancer mortality through screening (RR = 0.79; 95 % CI, 0.69–0.91) [5]. A sub-group analysis of prostate cancer mortality by age at randomization identified a significant decrease in prostate cancer mortality in the 65–69 year age group (RR = 0.69; 95 % CI, 0.55–0.87). No statistically significant difference in prostate cancer mortality was observed between screening and control groups in men aged <54, 55–59, and 60–64 years [5]. The results also suggest that screening is not beneficial in men aged over 70 years (RR = 1.17; 95 % CI, 0.82–1.66). The ERSPC study authors concluded that, “…the time for population-based screening has not yet arrived…” [5].

P Albertsen: Based upon what we know in 2015, PSA testing does benefit some men. For this reason I strongly support the recommendations of the AUA. These guidelines were based extensively on the data provided by the ERSPC. However, we need to do better. I am aware of your work with Hans Lilja, suggesting that a PSA value at age 50 is predictive of the long-term probability of developing clinically significant prostate cancer [18]. I believe we can refine the group of men who should be tested. We also need to incorporate the natural increase in PSA that comes with prostate enlargement that occurs when a man ages through his 50s and 60s. A graphic chart that tracks PSA levels or, possibly, percent free-PSA or the new prostate health index test against age and prostate volume, similar to a pediatric growth chart, might be helpful. Men consistently falling outside the 90th percentile, for example, might undergo MRI testing before considering a prostate biopsy. All of these refinements should be aimed at lowering the incidence of low-volume, low-grade cancer. In my mind, we have yet to define a best practice screening and treatment algorithm. While the AUA recommendations are a start, we have not agreed upon the test(s) needed, the frequency of their application, the value of imaging versus biopsies, nor on which treatments work best for which patients.

A new RNA test of blood platelets can be used to detect, classify and pinpoint the location of cancer by analysing a sample equivalent to one drop of blood.

Using this new method for blood-based RNA tests of blood platelets, researchers have been able to identify cancer with 96 per cent accuracy and classifying the type of cancer at an accuracy of 71 per cent.

“Being able to detect cancer at an early stage is vital. We have studied how a whole new blood-based method of biopsy can be used to detect cancer, which in the future renders an invasive cell tissue sample unnecessary in diagnosing lung cancer, for instance. In the study, nearly all forms of cancer were identified, which proves that blood-based biopsies have an immense potential to improve early detection of cancer,” according to Jonas Nilsson, cancer researcher at Umeå University and co-author of the article.

In the study, researchers from Umeå University, in collaborations with researchers from the Netherlands and the US, have investigated how a new method of blood-based RNA tests of the part of the blood called platelets could be used in detecting and classifying cancer.

The results show that blood platelets could constitute a complete and easily accessible blood-based source for sampling and hence be used in diagnosing cancer as well as in the choice of treatment method.

Blood samples from 283 individuals were studied of which 228 people had some form of cancer and 55 showed no evidence of cancer. By comparing the blood samples RNA profiles, researchers could identify the presence of cancer with an accuracy of 96 per cent among patients. Among the 39 patients in the study in which an early detection of cancer had been made, 100 per cent of the cases could be identified and classified.

In follow-up tests using the same method, researchers could identify the origin of tumours with a so far unsurpassed accuracy of 71 per cent in patients with diagnosed cancer in the lung, breast, pancreas, brain, liver, colon and rectum. The samples could also be sorted in subdivisions depending on molecular differences in the cancer form, which can be of great use in the choice of treatment method.

Scientists have developed a blood test that can identify key mutations driving resistance to a widely used prostate cancer drug, and identify in advance patients who will not respond to treatment.

The new research paves the way for information from a blood test to inform prostate cancer treatment in future, with only those patients whose cancers are free of resistance mutations taking the drug, abiraterone.

The study is also a proof of principle that tests for cancer DNA in the bloodstream can be used to detect drug resistance mutations – allowing patients who will not benefit from one drug to be given an alternative treatment instead.

Researchers at The Institute of Cancer Research, London, the Royal Marsden NHS Foundation Trust, and the University of Trento, Italy, analysed 274 blood samples from 97 patients using state-of-the-art DNA sequencing techniques.

They found that mutations in a gene called the androgen receptor (AR) predicted resistance to the prostate cancer drug abiraterone, and that patients with these mutations had poorer survival.

Abiraterone, which was discovered at the ICR, is now standard treatment for men with advanced prostate cancer – but while it is highly effective in many patients, 30-60% do not respond.

So researchers have been searching for a marker that will help predict in advance which men will benefit from the drug, and who should be given a different treatment.

Researchers discovered that men who harbour either a specific mutation or an increase in the number of copies of the AR gene, were 7.8 times less likely to have a reduction of more than 90% in their PSA levels, a widely used test to monitor the response of prostate cancer.

The study also found that in about 15% of men given abiraterone who did not have either mutation before starting treatment, this was acquired as the drug stopped working and appeared in the bloodstream several months before patients developed any symptoms.

Blood tests are particularly valuable in cancer patients because biopsies are often difficult to perform and can carry risks. Even when biopsies are possible, they only give a snapshot of cancer genetics in a small specific area, whereas blood tests can give information that is more representative of multiple different tumours around the body.

Dr Gerhardt Attard, Clinician Scientist at The Institute of Cancer Research, London, and Consultant Medical Oncologist at The Royal Marsden NHS Foundation Trust, said: “The discovery of abiraterone was an important step forward for patients with advanced prostate cancer, but we know it doesn’t work for all men, and we’ve been searching for a marker that will tell us in advance which men will benefit. We’re delighted to have developed a test that appears to predict very accurately whether a patient will respond to abiraterone, and that it can be performed on blood samples – removing the need to take a biopsy.

“We are now planning a clinical trial involving up to 600 men in which we aim to prospectively show that men who are positive with our test have significantly greater benefit from chemotherapy in preference to abiraterone or similar drugs. Critically, we believe that this sort of technology would be relatively straightforward to implement in NHS hospitals, making it accessible to a large number of patients. Additionally, looking at tumour DNA in the blood of patients could potentially give us an overall picture of why the cancer is progressing all over the body, unlike a biopsy that only tells us about the area sampled.”

Professor Paul Workman, Chief Executive of The Institute of Cancer Research, London, said: “Abiraterone has extended the lives of many thousands of men in the UK, with fewer side-effects than chemotherapy, and we are really proud that it was discovered here at the ICR. But we don’t stop at taking a new treatment like abiraterone to patients – it’s also essential to continue to conduct research on these new drugs, to make sure they are being used as effectively as possible.

“This new study finds that by analysing tumour DNA present in the bloodstream, we should be able to personalise treatment with abiraterone, so that only those who will benefit from the drug will receive it. It is the latest step forward in the new era we are in of precision cancer medicine, where rather than using a treatment in the hope that a patient will benefit, we can look at tumour DNA in advance and be much more confident that a particular treatment will work.”

Associate Professor Francesca Demichelis, leader of the Computational Oncology Group at the University of Trento, Italy, said:

“Being able to quantify tumor DNA from a blood sample and to characterize structural changes of the DNA that are predictive of treatment response is powerful. This is especially true as a blood-based test gives us a global picture of the patient’s tumor burden rather than a focal snapshot as with a standard biopsy.

“In principle the same computational approach we adopted in this study is applicable across tumor types. This study represents a step towards sensitive and specific detection of resistance to targeted therapies.”

“Abiraterone, a drug which Cancer Research UK helped to develop, gives men with advanced prostate cancer a much-needed treatment option and more time with their loved ones. But it doesn’t work for everyone. If these important early results bear up in larger clinical trials it could lead to a test which would indicate which patients might benefit more from trying other therapies instead.”

Dr Iain Frame, Director of Research at Prostate Cancer UK said: “We know that a one-size-fits-all approach to treating prostate cancer doesn’t work, and research like this is crucially important in helping us understand which treatments will – and won’t – work best for each individual. When the clock is ticking for a man with advanced prostate cancer, finding out early that his treatment needs changing can not only save precious time, but can also help avoid unpleasant side effects from a treatment that longer works for him.

“Research like this wouldn’t be possible without the thousands of people who take part in Movember every year – and so with another campaign upon us, we can all do something to help the fight against prostate cancer.”